Considerable efforts have been made in the petroleum industry to exploit offshore resources i.e. hydrocarbon oil and gas-bearing reservoirs which are accessible from a marine vessel or offshore installation via a riser connected to a subsea wellhead. Whereas many reservoirs have been accessible by coastal inshore or shallow water facilities, increasingly prospecting for such reservoirs is conducted in challenging deep marine environments. The facilities required for production and export of the hydrocarbons have become correspondingly complex and expensive. The literature on vessels and structures designed for this purpose is extensive.
Whereas the oil and gas resources have been typically produced under natural flow or artificial lift during the early life of the field, as the reservoir becomes depleted a cost of capital and operating expenditure versus recovery value assessment may lead to a need to discontinue operations whilst the reservoir retains significant volume of remaining potentially extractable hydrocarbon resources. Maintaining a dynamically positioned vessel on station above a wellhead for example is expensive in terms of capital and operating expenditure and involves risk to operating personnel and potential risk to the environment e.g. if a storm drives the vessel off station and damage to the riser occurs.
Therefore, at some stage, a cost and risk assessment may indicate that it is no longer viable to continue production from the subsea well with that vessel or installation and the well may be sealed and abandoned leaving unrecovered hydrocarbon reserves.
Accordingly, it would be of value to the industry to devise alternative means for continuing the production of hydrocarbon without requiring the presence of a dynamically positioned vessel, production platform or the like manned structure.
Achievement of this objective would also permit exploitation of small fields of relatively modest hydrocarbon reserves which according to traditional methods would be considered too small to exploit economically.